Greenhouses and agricultural tunnels provide environments that can be more favorable in which plants can grow. An important aspect to such structures is the covering material through which sunlight is provided to the plants inside. Various covering materials have been used including glass, rigid polycarbonate sheet, rigid polymethacrylate sheet, and flexible films. Agricultural films which are largely used in greenhouse culture or tunnel culture include soft films which are about 30 to 200 microns thick and comprise, as a base resin, polyvinyl chloride (PVC), branched low-density polyethylene (LDPE), ethylene-vinyl acetate copolymers (EVA), linear low-density polyethylene (LLDPE), etc. Of the flexible films polyvinylchloride and particularly polyethylene predominate due to low cost and adequate mechanical properties (tear resistance, elongation, etc). Weather resistance, antifogging properties, heat-retaining properties, and transparency are also particularly important for agricultural films. In addition to improvements in these properties, development of films having an extended duration of life before replacement is desired.
Polyethylene film is hazy, has reduced light transmittance and degrades rapidly under exposure to light. Even with light stabilizer additives lifetime is limited.
Acrylic films used as capping for vinyl window profiles, siding, or outdoor furniture have excellent transparency. However such films do not have adequate mechanical properties required for greenhouse or agricultural tunnel covering materials, because many acrylic films lack of flexibility and have weak tear strength. Accordingly acrylic films are typically used in films suitable for lamination on fixed substrates as protective layers (see U.S. Pat. Nos. 4,663,213, 4,141,935, 3,562,235, 3,843,753 and 3,812,205).
JP1999077939A discloses a greenhouse film made with a thermoplastic polyurethane coated on each side with a water-born coating composition containing an acrylic (co)polymer made with methylmethylacrylate.
The atmosphere within greenhouses or tunnels surrounded by an agricultural film is saturated with water vapor that evaporates from the soil or plants, and the water vapor condenses on the inner surface of a cold film as small droplets that cause fogging. Water droplets on the film not only greatly reduce the incident sunlight due to irregular reflection but the droplets fall on the plants resulting in frequent occurrence of diseases.
Antifog treatments desirably prevent adhesion of water droplets to the inner surfaces of the film, thereby assuring transmission of sunlight into the greenhouse or tunnel, raising the soil temperature and air temperature within the greenhouse or tunnel, accelerating photosynthesis of the plants, accelerating healthy growth of the plants, and controlling the occurrence of plant diseases.
Currently employed antifog treatments include incorporation of an antifogging agent (antifog additive) into a film-forming ethylene resin compound and coating of an antifogging agent on a film.
Antifogging coating agents comprising an inorganic hydrophilic colloidal substance and a hydrophilic organic compound have been disclosed in JP63-45432, JP63-45717, and JP64-2158. The inorganic hydrophilic colloidal substances include colloidal silica, colloidal alumina, colloidal Fe(OH)2, colloidal Sn(OH)4, colloidal TiO2, colloidal BaSO4, and colloidal lithium silicate, with colloidal silica and colloidal alumina most generally used. Suitable hydrophilic organic compounds include various nonionic, anionic or cationic surface active agents; graft copolymers mainly comprising a hydroxyl-containing vinyl monomer unit and from 0.1 to 40% by weight of a carboxyl-containing vinyl monomer unit or a partial or complete neutralization product thereof; and sulfo-containing polyester resins.
However, an antifogging film obtained by coating a soft plastic film with an antifogging agent has not yet been employed practically as an agricultural film for the following reasons. Because of their low surface energy, soft plastic films for agricultural use generally have poor wettability and adhesion when coated with surface active agents or hydrophilic polymeric substances which have been used as antifogging agents. This tendency is particularly notable with soft ethylene resin films of low polarity such as LDPE, EVA, and LLDPE films. Therefore, when an antifogging agent is spray coated with a power atomizer onto a soft ethylene resin film, the antifogging agent needs to be used in a large quantity and this increases cost, and a large amount of time is required for spray coating operations. Further, spray coating may not provide uniform coverage, leading to insufficient antifogging effects. Where an antifogging agent is applied using a coater, etc., a large quantity of a coating is consumed, and the coating speed cannot be increased, resulting in an increase of cost. In either case, the coated antifogging coating agent is washed away together with running water droplets due to poor adhesion resulting in a very short life for the antifogging properties. Furthermore, the coated film may undergo blocking due to the stickiness of the antifogging agent. As a result, it has been impossible to retain antifogging effects in a stable manner for a long duration of at least a year or more desirably several years. Most state-of-the-art agricultural films exhibit antifogging properties for only about 1 month.
Antifogging agents commonly incorporated into the films include nonionic, anionic and cationic surface active agents.
Other methods for providing antifogging properties to agricultural films, in addition to the coating method and incorporation method, include chemical modification of the ethylene base resin or the ethylene resin film surface by introducing a polar group, such as a hydrophilic group. This technique, however, entails high cost at the present time and is difficult to apply to agricultural films.
Under these circumstances, none of the state-of-the-art polyolefin-based agricultural films satisfies the requirements of long lasting antifogging properties. Thus the market needs a greenhouse film that has high light transmittance, good physical properties, good light stability and extended lifetime.
Food products are often packaged in polymeric film for ease in handling by customers, to provide sanitary protection of the food product from the environment, to provide an easier and more economical wrapping than paper, or to provide a better view of the food product. Various polymeric materials such as polyvinyl chloride, poly(ethylene/vinyl acetate) and irradiated polyethylenes have been used in the food wrap market with plasticized polyvinyl chloride (PVC) being widely used in the market.
In the field of packaging, where polymer films are used for covering or enclosing items of sale, film clarity or good optical properties are desired. For example, freshly cut meat and vegetables are often packaged in these films, and certain properties are desired of the film for it to perform satisfactorily as a food overwrap. Film should have good “see through” clarity so the product can be viewed. High gloss is desired to add “sparkle” to the packages for aesthetic appeal to the customers.
As food products such as meats, bakery items, vegetables, fruits, and the like as are packaged, they generally experience a temperature change from where they are packaged to where they are stored or sold. This temperature change can cause the formation of water droplets on the inside surface of the film (commonly called “fog”). These water droplets are usually objectionable because they hinder the view of the food products from the consumer and can cause spoilage, particularly to certain vegetables. There is a recognized need for polymer films that resist fogging when the interior of the package contains enough moisture to produce fogging when exposed to conditions of temperature and humidity which can produce condensation on the film.
To prevent water from condensing in droplets, an antifog agent is often added to the film. The antifog agents reduce the surface tension of the water and cause the water to form a continuous sheet, which is then transparent.
There is also a perceived need in the market place for improved packaging films, especially for food products, where the widely used plasticized polyvinylchloride (PVC) requires replacement by a more acceptable film. Much of the PVC which has been used is plasticized with additives, e.g. dioctyl phthalate, which are becoming less acceptable in the market.
U.S. Pat. No. 4,189,420 discloses certain ethylene polymers blended with a polybutene and a mixed glyceride having at least one acyl group of 2 to 6 carbon atoms and at least one acyl group containing 8 to 22 carbon atoms. U.S. Pat. No. 3,048,266 discloses an antifog agent of polyethylene oxide derivative in a polyolefin composition. U.S. Pat. No. 3,048,263 discloses a polyolefin antifog agent comprising a monoglyceride of a fatty acid. U.S. Pat. No. 2,462,331 discloses the incorporation into polyethylene of polyhydric alcohol esters or metal salts of either saturated or unsaturated monocarboxylic fatty acids.
U.S. Pat. No. 5,262,233 discloses agricultural films which may have incorporated therein an antifogging agent which may be a poly(ethylene oxide) of a long chain alcohol. U.S. Pat. No. 4,486,552 discloses fog-resistant packaging films having incorporated therein (1) an alkoxylated alkyl phenol along with (or in combination with) a mixed mono-, di- and/or triglyceride, or (2) a polyoxyalkylene fatty acid ester, or (3) a combination of (2) and any part of (1) above. U.S. Pat. No. 5,001,015 discloses polyolefin films with antistatic properties which include as possible antistatic agents the reaction products of polyalkoxylates with fatty alcohols.
U.S. Patent Application Publication 2003/0050373 discloses antifogging compositions comprising potassium neutralized fatty acid modified ionomers.
U.S. Pat. No. 7,037,964 discloses polyolefin films incorporating a compound of the formula CH3CH2(CH2CH2)aCH2CH2(OCH2CH2)bOH, where a is 9 to 25 and b is 1 to 10. The polyolefin films are said to be useful as fog resistant packaging films for moist products and greenhouse films for agricultural applications.
Atmer® 502, available from Uniquema, is described in a September, 1998 data sheet as having long-lasting antifog properties in LDPE agricultural film. Atmer® 502 is a 2 mole ethoxylated stearyl alcohol, C18H37(OCH2CH2)2OH.